Adam in my experience the best people to contact is the companies head office. Nine times out of ten they have a motability department who will give you the information they have. It is not always 100% but virtually always more accurate than the dealership or motability.
My own opinion is that it will be 2024 before we see any marked change in the number of high end manufacturers rejoining the scheme. The chip shortage is expected to last well into 2023 and until this is back to pre pandemic levels we will see no improvement.